The t-underdot characters occur in very few fonts, because they are
used in the standard orthography of very few, if any, languages. (They
most commonly occur in "scientific" transliterations of Semitic and
South Asian languages.) Underdotted vowels occur in the standard
orthography of a number of West African languages.
A font that has preformed characters for a very large number of
letters with diacritics (accents, etc.) is Gentium, created by the
Summer Institute of Linguistics, and available as a free download at
http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&item_id=Gentium
(if this doesn't show up as a link, just google Feeling Lucky "
gentium SIL " without the quotes).